1. Field of the Invention
The present invention relates to a linear encoding device which is used in a mechanical part performing a relative motion, for example, a machine tool, an industrial robot, and the like to perform a positional detection, and especially relates to a linear encoding device by which a long positional measurement can be performed and which is most suitable for being mounted on a long and high speed guiding device, and further which has a simple arrangement.
2. Description of the Prior Art
As a linear encoding device of the prior art, for example, there is a device disclosed in Japanese Patent Application Laid-Open NO. 5-340426. FIG. 1 is an explanation figure of the enlarged principal part of the linear encoding device of the prior art.
As shown in FIG. 1, this linear encoding device comprises a permanent magnet 1 which is a detected element on the movable side attached to a table (not shown in the figure) and moving in the longitudinal direction A, and magnetism electricity converting elements 2 which are detecting elements on the fixed side. The permanent magnet 1 comprises 8 poles of magnetizing where S poles and N poles are alternately arranged in the direction of movement shown by the arrow A. The magnetism electricity converting elements 2 are mounted in a line at specified intervals on a base board 3 on the fixed side, facing the permanent magnet 1.
The detection of the position and the detection of the direction by this linear encoding device are performed such that when the permanent magnet 1 moves in the direction shown by the arrow A, the resistance value of a magnetism electricity converting element 2 corresponding to the permanent magnet 1, changes with the change of the magnetic field, and in turn, the change of the magnetic field arises in the adjacent magnetism electricity converting element 2, and the similar changes of the resistance continue, and consequently, these changes of the resistance value are detected. Furthermore, the change of the resistance value becomes an output signal corresponding to the number of poles of the permanent magnet 1 in each magnetism electricity converting element 2, since the permanent magnet 1 is multipolarized and magnetized, and in this case, it comprises 8 poles of magnetizing. In this case, the change of the resistance value becomes an output signal of 4 cycles, since a signal of N pole and a signal of S pole are alternately output.
In this linear encoding device, EQU M=P-t
where the total length of the permanent magnet 1 in the direction A of movement is M, the arrangement pitch of each magnetic pole (N, S) of the permanent magnet 1 is t, and the arrangement pitch of each magnetism electricity converting element 2 is P.
A linear encoding device of the prior art has such an advantage that the magnetism electricity converting element 2 is mounted on the base board 3 which is the fixed side, and the detection signal can also be obtained from the fixed side, and therefore, the connection cable for the processing of the signal from the movable side is unnecessary, and the connection cable is not trailed. However, since the output signal of the magnetism electricity converting element 2 is made to be an analog sine wave signal corresponding to the multipolarized and magnetized permanent magnet 1, there is such a problem that for processing this sine wave signal, signal processing circuits such as an amplifier circuit or an arithmetic processing circuit are necessary so that the control device may become complex.
Furthermore, as shown in FIG. 1, a linear encoding device of the prior art has such a problem that the gap g between the permanent magnet 1 and the magnetism electricity converting element 2 should be small, and further, the fluctuation of the gap g should also be small, that is, the gap should be set to a gap of approximately 70 .mu.m.+-.10 .mu.m. The reason is that the pitch t of the magnetic poles of the permanent magnet 1 is a fine pitch, and therefore, if the gap g is large, the discrimination of the adjacent magnetic poles of the permanent magnet 1 becomes unclear, and the detecting ability of the magnetism electricity converting element 2 cannot perform a detection with a high accuracy.
Furthermore, in a linear encoding device of the prior art, the pitch t of the magnetic poles of the permanent magnet 1 is fine, and the discrimination power is also highly accurate, but there is such a problem that to make the operation faster is difficult. The reason is that similarly to the above mentioned problem of complexity, the processing capacity for the signal processing of the analog sine wave signal from the magnetism electricity converting element 2 by the signal processing circuit, is large, and the signal processing in a short time is difficult.